The invention relates to a method and apparatus for measuring the pressure, in particular the static or quasi-static pressure, of a liquid. The invention has particular application to a measuring apparatus, and to a method using such apparatus, of the kind including a measuring piston which is guided in a cylinder and is acted upon by a pressure dependent on the pressure to be measured and which is in communication with a load measuring device, wherein the piston is guided in the cylinder in a substantially frictionless manner by virtue of lubrication provided by a separating fluid supplied under pressure. Such a measuring apparatus, hereinafter referred to as a measuring apparatus of the kind specified, is disclosed in U.K. patent application No. 8231524, Publication No. 2111212A. A measuring apparatus of the kind specified, and a method using such apparatus, may be used, for example, for measuring the mass of contents of a storage tank for pretroleum products.
When measuring the pressure of a liquid, in particular that of a liquid having a low boiling point, such as propane, butane and the like, there is encountered the difficulty that vapor or gas pressure loading the liquid level from above in a closed tank influences the measurement. As vapor pressure is subjected to variations depending upon temperature, the measurement of the pressure exerted by a column of liquid loaded by vapor pressure may produce unreliable results when using a measuring apparatus of the kind specified. Even if the vapor pressure is ascertained with a view to eliminating the effect of the vapor pressure, the method would be complicated and still not very accurate.